This invention relates to a limiter amplifier which is suitable for constituting a direct conversion system FSK (Frequency Shift Keying) signal receiver having excellent sensitivity and band characteristics.
A practical FSK signal receiver, such as is used in a pager, has most of its parts constructed to form a direct conversion system which can be integrated on an integrated circuit (IC) for small size and low cost of the apparatus. The limiter amplifier is used for a FSK receiver which employs an intermediate frequency.
FIG. 3 shows the construction of a first conventional limiter amplifier. In FIG. 3, there are shown amplifiers 11 and coupling capacitors 12. The amplifiers 11 are cascaded through each of the coupling capacitors 12.
FIG. 4 shows the construction of a second conventional amplifier. In FIG. 4, there are shown differential-input differential-output amplifiers 1, feedback resistors 2, 4, 5 and bypass capacitors 3, 6. The amplifiers 1 are differentially DC-connected, and the output of the series of amplifiers is fed back to the input through the feedback resistors 2, 4 and 5.
However, since the conventional limiter amplifiers given above obtain a large gain in the intermediate frequency band, it is difficult to obtain a large gain in the low frequency band which is necessary in a direct conversion system limiter amplifier In other words, in the conventional example shown in FIG. 3, the capacitance of the coupling capacitors 12 must be increased in order that a large gain can be assured in the low frequency band, and thus it is difficult to incorporate such coupling capacitors in an IC. Moreover, since the conventional example shown in FIG. 4 is able to obtain a large gain over a frequency range including and above the low frequency band necessary in a direct conversion system, the S/N ratio of a desired signal is decreased. It is desirable that a limiter amplifier used for a direct conversion system have a large gain only within a necessary low frequency band (for example, 1 kHz to 10 kHz).